A citizens group is suing to block state plans to convert U.S. 89 into a full freeway in Davis and Weber counties to connect Interstates 15 and 84 — a project that is scheduled to begin next year and cost $275 million.

Residents’ Voices United on 89 (REVU89), a group of 1,500 residents, contends the Utah Department of Transportation used the wrong procedures and criteria in an environmental study that led to the decision to proceed with the project.

In a lawsuit filed in 3rd District Court, it said UDOT improperly used new state criteria that have never been legally adopted, instead of following the tougher National Environmental Policy Act — which the group contends should have been used because the project ties into federal interstate freeways.

REVU89 also says the state improperly limited its environmental study to just the U.S. 89 corridor while it says it should have conducted a wider, regional traffic study to include the nearby parallel I-15 and West Davis Corridor.

The group has said the narrower focus excluded close consideration of other reasonable alternatives — such as widening I-15 instead.

Also, northbound I-15 now provides no direct connection to eastbound I-84 where they intersect, and residents have contended that providing it would prevent the need of converting U.S. 89 into a freeway as the only long connection between the freeways.

REVU89 fought the proposal during earlier public hearings saying that placing a freeway in what is now almost an entirely residential area will ruin communities by attracting a swarm of businesses to interchanges.

Also, it argues that Kaysville soon will have three parallel freeways — I-15, U.S. 89 and the new West Davis Corridor — separated by only 4.5 miles, which they argue is poor planning, and contend it would be wiser to just widen I-15.

UDOT issued a written statement Thursday about the lawsuit that said its environmental study was “thorough and collaborative” and used a “rigorous process” with “numerous public meetings and opportunities for public input.”

UDOT said it incorporated many ideas of residents into the final plan, and “we will continue the collaborative effort as we move forward with project plans, and remain committed to providing an appropriate transportation solution for the traffic demands on the U.S. 89 corridor.”

UDOT plans to widen U.S. 89 from four to six lanes and convert it into a freeway between Shepard Lane in Farmington and I-84 in South Weber. U.S. 89 already essentially is a freeway between Shepard Lane and I-15.

UDOT has said that without the project, congestion would increase by 20 percent to 35 percent on the corridor by 2040 — creating peak travel times of one hour from Shepard Lane to I-84. With planned improvements, that same trip in 2040 is projected to take nine to 10 minutes.

The contested study and an interactive map of the project are available online at udot.utah.gov/us89.